The aim of this research is to determine the biophysical mechanisms of sensory transduction and encoding in identified mechanoreceptors of the vertebrate atrial endocardium. Though the sensory discharge patterns of these receptors in vivo are well known in relation to circulatory events, and a rich variety of morphologcal types have been described, their excitatory properties remain poorly understood, and the correspondence between form and function has not been established. We have developed an in vitro atrial preparation which showed considerable promise in an exploratory study of threshold, sensitivity, adaptation and directionality in relation to deformation of the endocardial surface, and we propose to extend this work with four new or recently improved techniques: (1) A compliance microprobe, developed in our insect mechanoreceptor work, to determine the role of mechanical properties of the endocardial surface in patterning the sensory discharge; (2) Computer-assisted analysis of the sensory code using controlled stimuli; (3) Perfusion with altered salines and drugs to explore the ionic bases of transduction; (4) Backfilling with cobalt and related intracellular stains in an effort to identify the sensory ending morphologically after it has been characterized physiologically.